Rett Syndrome is a form of severe mental retardation. The overall goal of this proposal is to define the neurodevelopmental defects in Rett Syndrome using the olfactory system as a model. Although Rett Syndrome has a genetic basis, the neuronal defect(s) in Rett Syndrome are unknown. As with many neurological disorders, our understanding of Rett Syndrome is hindered by the inaccessibility of brain tissue during the period of disease progression. Olfactory receptor neurons (ORNs) are continually replaced during life from a population of basal cells, recapitulating their developmental program. They are accessible to low-risk biopsies. Thus, ORNs provide a unique model for Rett Syndrome. Our results obtained from nasal biopsies of Rett Syndrome patients and age-matched controls confirm the feasibility and usefulness of this multidisciplinary approach. We hypothesize that ORN differentiation is arrested by a non-cell-autonomous mechanism that disrupts dendritic development and initiates apoptosis. Aim 1 will analyze olfactory biopsies of Rett and age-matched controls by immunohistochemistry and electron microscopy using stage-specific markers to determine the phenotype and the developmental stage of arrested ORNs. TUNEL staining and PCNA are used to evaluate cell death and compensatory neurogenesis. The expression and distribution of proteins whose involvement in pathogenesis is suggested by high density arrays is studied. The specificity of the morphological changes seen in the olfactory epithelium of Rett is determined by comparisons to biopsies from Fragile X, Down's Syndrome, and autism. Aim 2 will evaluate the olfactory bulb, the only target of ORNs. The simple circuitry and ongoing synaptogenesis in the olfactory bulb make it an excellent model to determine the consequences of this arrest on the morphology of mitral cells axons and dendrites, and their connectivity to ORNs. We will use immunocytochemistry, retrograde labeling techniques, and tyrosine hydroxylase staining. Aim 3 will assess the functional consequences of the defects seen in ORNs and the bulb by odorant-stimulated functional MRI. Aim 4 will exploit our extensive experience with ORN culture and modeling of physiologically-relevant factors required for ORN survival. We will investigate the ability of in vitro modulations to ameliorate ORN phenotype in cultures isolated from Rett biopsies compared to age-matched controls. These experiments will test our hypothesis that Rett is caused by a non-cell-autonomous mechanism from which ORNs can be rescued in vitro. Thus, understanding the primary and secondary defects in Rett Syndrome will provide insight into an important aspect of post-natal brain developmental plasticity.